Sweet Vernal grass grows primarily as a winter annual and occasionally as a perennial cool-season grass. It gives off a distinctive, sweet-smelling odor when mature which can give hay a sweet smell.
However, this plant has little nutritional value and can actually become a weed of hay fields and pastures. It grows approximately 2 feet tall and produces spikes of brownish-yellow seed heads in April and May. It is one of the earliest grasses to produce a seed head in our area.
Under certain conditions sweet vernal grass in hay can be toxic to livestock. It contains coumarins similar to a chemical compound found in sweet clovers. This is what gives the plant its sweet vanilla-like odor. When grass hay is processed or exposed to conditions of excess moisture, coumarin converts to a toxic metabolite by fungal-type microorganisms.
Therefore, the risk for toxicity is most likely when moldy hay that contains large quantities of sweet vernal grass is fed to livestock. Fields containing large quantities of sweet vernal grass can still be harvested for hay, but measures should be taken to minimize the risks of hay to become moldy.
If bales develop a potential problem with molds and it is still necessary to use them as a feed source, then dilute contaminated hay with alternative hay sources that do not contain sweet vernal grass or use other feed supplements.
High populations of sweet vernal grass are most likely an indicator of poor stands of the desirable cool-season forage grasses. Field conditions that contain thin forage stands provide an opportunity for sweet vernal grass to germinate, particularly during the fall and late winter months.
Control strategies such as clipping field areas containing sweet vernal grass before seed heads can mature are not feasible. The best long-term solution is to implement management practices that will thicken the stand of desirable forage species such as interseeding or use of renovation methods.
In many cases, improving the soil fertility will improve the competitiveness of the desirable cool-season grasses against this plant species. The first thing on my soil fertility check list is pH, which measures the acidity of the soil.
Ideally, we would like to see our grassland soils slightly acid, with a pH around 6.4, with 7.0 being neutral and lower pH’s being more acidic. It is not uncommon for soil pH to be around 5.4, which is a soil acidity level that will tend to favor non-productive grasses like sweet vernal and not more productive grasses like orchardgrass.
To raise soil pH, apply lime. How much? That depends on your current soil pH and you don’t know that without a soil test. The WVU soil testing lab analyzes samples free of charge and mailer kits are available at the Extension Office.